1. Field of the Invention
This invention relates to a method and apparatus for detecting the presence of live 3-phase electrical lines, one electrical line for each phase, inside a buried pipe without penetrating the pipe wall.
2. Description of Related Art
Buried high-voltage, 3-phase electrical lines are frequently installed inside pipe that is the same as or similar to pipe utilized for natural gas transmission and distribution. Voltages may be as high as 70,000 volts. Normal practice of the electric utility is to fill the space between the electrical lines and the inner pipe wall surface with either a dielectric fluid or nitrogen gas, typically at 200 psig. Because the pipes used to contain the 3-phase electrical lines are visually indistinguishable from the pipes used for natural gas transmission and distribution, some method for distinguishing between the pipes without penetrating the pipe walls is necessary. Presently, when such pipes are buried near each other, utility maps and electromagnetic pipe locators are used to identify which pipe is which. However, the results of these methods are sometimes incorrect. As a result, incidents have occurred where a pipe containing electrical lines has been drilled into by utility crews intending to attach a new service to a gas main. The potential for tapping into misidentified, electrically energized pipelines causes undue risks and hazards to employees, the general public, and the environment.
At present, there are no acceptable tools for determining pipe contents from outside the pipe. Drill holes can be made, but doing so is hazardous because the pipe contents are under pressure and live electrical cables may be inside. X-ray inspection to detect electrical cables has been tried, but such technique does not provide an indication as to whether the electrical cables are live. In addition, the use of X-ray equipment in the field is not an acceptable solution for utility companies.
The most efficient transmission of electricity occurs when the three electrical lines are twisted around each other and the current flows in each line are balanced with the proper phases to cancel the electric and magnetic fields outside of the pipe carrying the electric lines. However, balanced currents means that techniques for identifying the utility of the pipe that measure electric and magnetic fields are not effective. Thus, it is apparent that an easy to use, inexpensive, externally applied tool is needed to detect electrical cables inside a pipe.